The office workspace has undergone significant changes in the last 30 years where work areas have become smaller with increasing emphasis on collaboration.
Sound control is a vital aspect of worker efficiency. Significant effort is expended in the design of the workspace in order to control the acoustics, reducing the environmental noise that interferes with a worker's concentration. In addition, public spaces are often plagued with environmental noise. It is desirable to reduce the perception and effect of environmental noise in public areas such as airports, subways, and trains.
Historically, sound control has been through the deployment of passive means such as large separation distances, acoustical ceiling tile, carpeting, partitions, and other absorptive materials to reduce the sound waves as they propagate from the source to a listener.
However, in the contemporary design standards, the distance between workstations is being substantially reduced. In addition, interior design is seeking to remove many of the absorption surfaces to create a cleaner environment. All of these elements are collaborating to create an acoustic environment where it is difficult to achieve optimal worker efficiency.
Office designers have noted that the noise level is not necessarily distracting. What has been determined is most distracting are those sounds that attract attention such as conversation between two or more people, fragments of telephone conversations, personal acoustic eruptions, etc. The attention attractor is the information content.
A further advance in office noise control is the addition of sound in the form of filtered white noise. The noise is shaped to decrease the signal to noise ratio of the distracting sound to the point where the sound is no longer intelligible and hence distracting. In this application, the speakers are typically mounted in the plenum between the acoustic ceiling and the overhead. The speakers are acoustic point sources where the projected sound has directionality that is frequency dependent. Effective coverage of masking sound is difficult in that the ideal application is one where the sound transmitting through the acoustic ceiling is uniform and of the correct spectral content. The basic sound characteristics of the sources make this a difficult task. Further complicating the matter is that the acoustic point sources typically need to operate at higher levels to overcome the acoustic absorption of the ceiling.
A more recent development in noise masking is the generation of acoustic babble. (US Patent Application Publication No. US/2005/0065778A1) In this process, a person's voice is processed by an electronic signal processor which randomly inverts, time delays and then feeds the processed signal to an audio speaker. The resulting acoustic signal substantially reduces the intelligibility of speech to where it is no longer a distraction to a worker within the original speaker's acoustic field.
U.S. Pat. No. 6,904,154 teaches that optimal performance of a distributed mode loudspeaker includes a member extending transversely and capable of sustaining bending waves over an area of the member. The member having a distribution of resonant modes of its natural bending wave vibration dependent on specific values of particular parameters, including geometrical configuration and directional bending stiffness(es). The values have been selected to predetermine the distribution of natural resonant modes consonant with required achievable acoustic action for operation of the device over a desired operative acoustic frequency range.
The distributed mode loudspeaker of the '154 patent is impractical in a built environment having structures and furnishings that rarely fall within the design parameters of the described distributed mode loudspeakers. In addition, the placement of the inertial type transducer is determined by factors related to optimal acoustic placement, but not relative to aesthetics, tampering, and convenience in installation and maintenance.
US Pat. Application No. US2006/0147051 A1 teaches inertial transducers of the magnetostrictive form using Giant Magnetostrictive Materials (GMM) as the active element. These types of inertial transducers have limited low frequency performance, excessive distortion and limited overall displacement. Mechanical engineering efforts to increase low frequency performance come at the expense of additional distortion. The limited displacement of the GMM based inertial transducer also restrict their application to panels or structures that are relatively stiff, thus not making them suitable for many other built environment surfaces. The '05 application teaches the use of a controller mixer for comparing ambient noise or other signal to control the acoustic output of the overall system or cause notification of other engagement with the system. The patent does not address configuring the signal for optimal acoustic response of the driven structure to improve audio fidelity to the input signal. Further, the application teaches that the invention can be used for anti-noise control but fails to address how a spatially incoherent acoustic source can create a coherent anti-phase signal for active noise cancellation.